codegen_llvm: avoid Deref impls w/ extern type

`rustc_codegen_llvm` relied on `Deref` impls where `Deref::Target` was
or contained an extern type - in my experimental implementation of
rust-lang/rfcs#3729, this isn't possible as the `Target` associated
type's `?Sized` bound cannot be relaxed backwards compatibly (unless we
come up with some way of doing this).

In later pull requests with the rust-lang/rfcs#3729 implementation,
breakage like this could only occur for nightly users relying on the
`extern_types` feature.

Upstreaming this to avoid needing to keep carrying this patch locally,
and I think it'll necessarily need to change eventually.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -637,7 +637,7 @@ pub(crate) unsafe fn llvm_optimize(
     let result = unsafe {
         llvm::LLVMRustOptimize(
             module.module_llvm.llmod(),
-            &*module.module_llvm.tm,
+            &*module.module_llvm.tm.raw(),
             to_pass_builder_opt_level(opt_level),
             opt_stage,
             cgcx.opts.cg.linker_plugin_lto.enabled(),
@@ -875,7 +875,7 @@ pub(crate) unsafe fn codegen(
             };
             write_output_file(
                 dcx,
-                tm,
+                tm.raw(),
                 config.no_builtins,
                 llmod,
                 &path,
@@ -909,7 +909,7 @@ pub(crate) unsafe fn codegen(
 
                 write_output_file(
                     dcx,
-                    tm,
+                    tm.raw(),
                     config.no_builtins,
                     llmod,
                     &obj_out,